Bragg gratings have a number of important applications in optical components, such as, lasers, sensors, dispersion compensation devices, etc. There are many ways to manufacture Bragg gratings on an optical fiber or in a planar waveguide. A Bragg grating comprises a number of partially reflecting elements imprinted or embedded onto the light guiding element (e.g., optical fiber or optical waveguide). While the manufacturing of Bragg gratings by using holographic exposure of optical fiber has been well established, the manufacturing of Bragg grating in planar waveguides has encountered a number of challenges, including unwanted parasitic gratings, as will be described further below.
Implementation of Bragg gratings in planar waveguides, known as planar Bragg gratings or PBG, has multiple advantages for monolithic as well as hybrid integration with other devices. Examples include monolithic integration of PBG with light amplification devices within a PLC device, rare-earth-doped waveguide lasers, etc. PBG can also be integrated with active components that are flip-chip mounted on the PLC device or otherwise coupled to the PLC device, for example, to form external cavity lasers (ECL) on a PLC platform.
The standard fabrication of PBG may use a silica-on-silicon optical bilayer as a PLC platform. In this process, first a buffer silica layer is deposited on a silicon wafer, followed by the deposition of a germanium-doped silica layer, which is called the ‘core’ layer. In case of PBG, grating element formation step is integrated into the waveguide formation process. In case of PLC, these grating elements are implemented by locally changing the index of refraction and/or geometry of the waveguide via holographic UV exposure of the germanium-doped silica core layer. The period of the grating structure defines the central wavelength of the resulting Bragg grating. For example, for a 1550 nm wavelength, the physical period of the Bragg grating corrugations is approximately 0.5 micron, i.e. each trench and each raised feature of the Bragg grating is approximately 0.25 micron. These features can be produced by means of an additional photolithographic and grating etch step on the germanium-doped core layer before etching the entire waveguide. After the PBG is defined, the waveguide itself is defined by means of another photoresist-based lithography step and an etch step (using reactive ion etching or RIE process or other etching processes). Waveguide sidewalls are exposed in this etching step. The process is completed by depositing a final layer of silica, i.e., the ‘cladding’ layer that encompasses the core layer.
During the process of etching the waveguide after the Bragg grating corrugations have been defined in the core layer, an out-of-phase “parasitic” grating is formed on the sidewalls of the waveguide, as shown in FIG. 1. FIG. 1 is a scanning electron microscope (SEM) picture, showing the top view of an etched waveguide before the cladding layer is deposited. As shown in FIG. 1, the individual grating corrugations are about 240 nm, and a physical period of the corrugation is about 540 nm. These parasitic features on the sidewalls are 180 degrees out-of-phase with the primary desired Bragg grating corrugations at the top of the waveguide. It is believed that the cause of the formation of the parasitic gratings is an interaction of the reactive ion etching reactants with the non-uniform photo resist layer used to mask the corrugated wave guide during waveguide etch. Note that the photoresist is not applied on a planar substrate, as the top surface of the core layer is already corrugated due to the presence of the PBG. The photoresist layer has a variable thickness due to the corrugated substrate it is applied on. SEM analysis revealed that the parasitic gratings are structures that may have a surface area approximately double that of the surface area of the primary gratings. Characteristics (e.g., depth, duty cycle, surface roughness etc.) of the parasitic gratings are largely uncontrollable. The out-of-phase parasitic grating has the effect of weakening or canceling out altogether the reflectance of the primary grating etched at the top of the waveguide, leading to uncontrolled optical performance of the Bragg grating, and the PLC device.
In addition to the problem of sidewall parasitic grating affecting the primary grating performance, a related problem observed during the fabrication of PBGs is outdiffusion of dopant from the core area to adjacent cladding or buffer materials during high-temperature (e.g., 800-100° C. or higher temperature) annealing. In case of germanium-doped silica core, the outdiffusion problem is particularly prominent, because at high annealing temperature, germanium can diffuse to distances on the order of 1 micron or more depending on the cladding material, which may result in erasure of refractive index variation created by the etched grating corrugations, if the outdiffusion is not prevented in certain directions.
Therefore, what is needed is methods of suppressing the formation of and detrimental effects of undesirable parasitic gratings on waveguide sidewalls in a PLC device with integrated Bragg gratings. In addition, it is desirable that the methods also address the problem of dopant outdiffusion from the core area of the waveguide in certain directions that negatively affects primary grating performance.